1. Field of the Invention
The present invention relates to a CDMA receiver, and in particular to a method of searching for paths by calculating the delay profile at prescribed intervals, this delay profile showing the power distribution of the received signal as a function of delay.
2. Description of Related Art
In mobile communications, the spread in the propagation path lengths of the signal received in a multipath environment means that there are multiple waves having different propagation delays. A feature of direct sequence code division multiple access (DS-CDMA) is that it is capable of separating and extracting these multiple waves in the form of a so-called “delay profile”, by using a wide-bandwidth spreading code to spread the narrowband data. It is well known that in DS-CDMA communications a diversity effect can be obtained and receiving characteristics improved by rake combining this plurality of multipath signals with different propagation delays. More specifically, a DS-CDMA receiver employs a multipath searcher to measure the delay profile (the distribution of signal power as a function of delay); selects several paths within the measurement range that have a high signal power; allocates these paths to respective fingers; and combines the selected paths in a rake combining receiver.
However, because the relative position of a mobile station changes relative to a base station, the radio wave transmission environment around the mobile station varies and hence the delay profile also changes. For this reason, the delay profile measured by the multipath searcher also varies, which results in variation in the delay associated with a path to be rake combined. Hence in a mobile communications environment it is necessary to follow these changes in the delay profile and to perform rake combining in such a way that maximum signal power is obtained.
In order to follow these delay profile changes described above, the path search method employed by a CDMA receiver to search for a path from the delay profile has hitherto had a multipath search function for detecting a path from the delay profile, plus a tracking function for tracking the provisionally detected path.
The delay profile is formed by calculating the correlation power at each of a plurality of delays by obtaining the correlation between the received data and the spreading code. However, because there is generally a large amount of data and always a considerable amount of calculation required to obtain this correlation, a conventional path search method that performs the same correlation computation for all timings within the delay profile measurement range has inevitably taken considerable time to perform the processing required to detect paths from the delay profile, and its implementation has been associated with high power consumption.
A problem encountered with the conventional CDMA receiver described above is that because an enormous amount of computation is required in the path search processing for discovering valid paths from a delay profile that has been obtained, both the amount of calculation and the power consumption are large.